1. Field of the Invention
The present invention relates to frequency converter used in a radio communication apparatus or the like for performing frequency conversion.
2. Related Background Art
A non-linear element such as a semiconductor diode and FET has been used for a conventional frequency conversion device. The purpose of the frequency conversion is to input a signal having a certain frequency into a frequency conversion device and output a signal having a frequency component different from that of the input signal.
The case of applying a current into a non-linear resistance r(i) will be discussed as a simplest example of performing the frequency conversion using the non-linear element. The current-voltage characteristic of the non-linear resistance can be provided with the Taylor expansion for x=i−i0 around an operation point (i0,v0) as the following expression.
                                                                        v                ⁡                                  (                  x                  )                                            =                                                i                  0                                +                                                                            (                                                                        ⅆ                          v                                                                          ⅆ                          i                                                                    )                                                              i                      =                                              i                        0                                                                              ⁢                  x                                +                                                      1                    2                                    ⁢                                                            (                                                                                                    ⅆ                            2                                                    ⁢                          v                                                                          ⅆ                                                      i                            2                                                                                              )                                                              i                      =                                              i                        0                                                                              ⁢                                      x                    2                                                  +                …                                                                                        =                                                i                  0                                +                                                      a                    1                                    ⁢                  x                                +                                                      a                    2                                    ⁢                                      x                    2                                                  +                                                      a                    3                                    ⁢                                      x                    3                                                  +                …                                                                        [                  Formula          ⁢                                          ⁢          1                ]            
When a current expressed by a sine wave of a frequency ω asx=mcos ωt  [Formula 2]is applied into such a non-linear resistance element, a voltage generated across both ends of the element becomes
                                              ⁢                  [                      Formula            ⁢                                                  ⁢            3                    ]                                                                    v        =                              i            0                    +                                    a              1                        ⁢            m            ⁢                                                  ⁢            cos            ⁢                                                  ⁢            ω            ⁢                                                  ⁢            t                    +                                    1              2                        ⁢                          a              2                        ⁢                                          m                2                            ⁡                              (                                  1                  +                                      cos                    ⁢                                                                                  ⁢                    2                    ⁢                    ω                    ⁢                                                                                  ⁢                    t                                                  )                                              +                                    1              4                        ⁢                          a              3                        ⁢                                          m                3                            ⁡                              (                                                      3                    ⁢                    cos                    ⁢                                                                                  ⁢                    ω                    ⁢                                                                                  ⁢                    t                                    +                                      cos                    ⁢                                                                                  ⁢                    3                    ⁢                    ω                    ⁢                                                                                  ⁢                    t                                                  )                                              +          …                                    (        I        )            and it is possible to take out higher harmonic components such as 2ω and 3ω in addition to the ω component proportional to the input current utilizing output waveform distortion.
Next, there will be discussed another case in which a signal input into the non-linear element is the sum of signals having two frequencies ω1 and ω2 different from each other. When an input current isx=ma cos ωat+mb cos ωbt  [Formula 4]a voltage generated across both ends of the non-linear resistance element becomes
                    v        =                              i            0                    +                                    a              1                        ⁢                          m              a                        ⁢            cos            ⁢                                                  ⁢                          ω              a                        ⁢            t                    +                                    a              1                        ⁢                          m              b                        ⁢            cos            ⁢                                                  ⁢                          ω              b                        ⁢            t                    +                                    1              2                        ⁢                          a              2                        ⁢                                          m                a                2                            ⁡                              (                                  1                  +                                      cos                    ⁢                                                                                  ⁢                    2                    ⁢                                          ω                      a                                        ⁢                    t                                                  )                                              +                                    1              2                        ⁢                          a              2                        ⁢                                          m                b                2                            ⁡                              (                                  1                  +                                      cos                    ⁢                                                                                  ⁢                    2                    ⁢                                          ω                      b                                        ⁢                    t                                                  )                                              +                                    a              2                        ⁢                          m              a                        ⁢                          m              b                        ⁢                          {                                                                    cos                    ⁡                                          (                                                                        ω                          a                                                +                                                  ω                          b                                                                    )                                                        ⁢                  t                                +                                                      cos                    ⁡                                          (                                                                        ω                          a                                                -                                                  ω                          b                                                                    )                                                        ⁢                  t                                            }                                +          …                                    [                  Formula          ⁢                                          ⁢          5                ]            and it is possible to take out a sum and a difference of the input signal frequencies; the sum (ω1+ω2) and the difference (ω1−ω2) In particular, a device obtaining the sum of the frequencies and a device obtaining the difference of the frequencies are called an up-converter and a down converter, respectively.
The generation of a signal having a frequency different from that of an input signal in this manner is called frequency conversion. The frequency conversion taking out the double frequency or integer multiple frequency of an input signal having a certain frequency as in Formula (I) is particularly called frequency multiplication, and the frequency conversion in the present invention includes the frequency multiplication.
The frequency conversion is an important technology. For example, the frequency conversion device is used for frequency mixing in a transmitter and a receiver in the radio communication field. Further, for the generation of a millimeter wave and sub-millimeter wave, a reasonable oscillator directly generating a signal having these frequency bands is not available and the generation is performed by a combination of a microwave oscillator and the frequency multiplication device.
Generally, the non-linear element used for the frequency conversion mainly utilizes a non-linear characteristic exhibited by a semiconductor device such as a diode and an FET. A schottky diode is frequently utilized as the frequency conversion device used for a microwave integrated circuit (MIC) which is formed by mounting discrete components on a dielectric substrate. Further, for the frequency conversion device used for the purpose of the frequency multiplication, the diode with reverse bias application is frequently utilized as a non-linear capacitance element (varactor).
There is known a monolithic microwave integrated circuit (MMIC) which is realized by collective and integrated fabrication of an active element, a passive element, and a passive-active element etc. on the same substrate by use of a semiconductor process. This MMIC uses an FET for an active device such as an amplifier and an oscillator, and thereby it is difficult to incorporate a diode designed dedicatedly for the frequency conversion into the MMIC from a restriction of production process compatibility. Therefore, the frequency conversion in the MMIC is frequently performed by the utilization of the non-linear characteristic of the FET itself. Further, when the frequency conversion device is incorporated in the MMIC, a circuit area is restricted from the view point of integration degree. Accordingly, the frequency conversion device is also expected to have a small scale.
The MMIC is broadly divided into an MMIC configured with a Si-series device and an MMIC configured with a compound semiconductor device. Although each of the Si-series device and the compound semiconductor device has both advantages and disadvantages, it is difficult to mixedly mount these devices on the same substrate in the monolithic microwave integrated circuit (MMIC). This is because a silicon substrate is used for the Si-series MMIC and a substrate such as GaAs is used for the compound semiconductor, since epitaxial growth is frequently necessary for a film deposition process for each of the devices. Production process compatibility is very poor between the Si-series device and the compound semiconductor device.
Further, in the frequency conversion device using a semiconductor, the frequency conversion device itself generally does not have a frequency selection capability. Accordingly, when trying to perform the frequency conversion only for a certain frequency, it is necessary to provide a filter or the like. In the frequency conversion device using the semiconductor, the frequency conversion itself cannot have a switching function.
Meanwhile, the giant magneto-resistance element (GMR) and the tunnel magneto-resistance element (TMR), which exhibit the magneto-resistance effect, have been developed for the application to a sensor or a memory device. Such an element utilizes a phenomenon that the resistance of a magneto-resistance element changes according to a relative angle of a magnetic moment between a magnetization free layer and a magnetization pinned layer in the magneto-resistance element. That is, the above giant magneto-resistance element or the tunnel magneto-resistance element utilizes a feature of detecting an external magnetic field change as a resistance change (sensor effect) and a feature of having resistance hysteresis reflecting magnetic hysteresis (memory effect). Further, recently there has been developed a device application utilizing spin injection torque in addition to the magneto-resistance effect. The spin injection torque is magnetic torque generated in a localized magnetic moment when angular momentum exchange occurs between a conduction electron and a localized electron by a spin-polarized current in a ferromagnetic body, as described in Non-patent document 1. Thereby, an application such as a microwave oscillator, a microwave detection device, and a microwave amplifier is being developed by the utilization of spin injection magnetization reversal in which magnetization reversal becomes possible without an external magnetic field or by the utilization of a non-linear effect caused by a precession movement of magnetization induced by the spin injection torque (refer to Patent document 1).
The operation principle of the microwave detection device shown in Non-patent document 3 is a homodyne detection method and it is possible to detect a DC voltage for one input AC signal. This device utilizes a non-linear effect in which the precession movement of the magnetic moment is caused by the spin torque induced by the AC signal applied to the magneto-resistance element, and the resistance value thereof changes periodically. The frequency of the resistance value change is equal to the frequency of the input AC signal and the effect expressed by Formula (I) appears. In Non-patent document 3, while the homodyne detection is performed utilizing this effect, another important technique is used. The technique utilizes a spin injection FMR effect. A minute AC signal provides a very small current value and also a very small induced precession movement of magnetization, and thereby an output DC voltage is very small. However, the magnetization precession movement is amplified by a resonance effect when the frequency of the input AC signal becomes close to a ferromagnetic resonance frequency. Thereby, it becomes possible to detect a larger DC voltage. The detection function using this magneto-resistance element is called a spin torque diode effect. In this manner, the ferromagnetic resonance is also caused by the spin injection torque and a sufficient non-linear effect can be obtained in the magneto-resistance element by the additional use of the ferromagnetic resonance, and thereby an application is expected in the microwave band.
Prior Art Document
Patent Document
Patent document 1: Japanese Patent Laid-Open No. 2006-295908
Non-Patent Document
Non-patent document 1: Slonczewski, J. C. Current-driven excitation of magnetic multilayers. J. Magn. Magn. Mater. 159, L1-L7 (1996).
Non-patent document 2: Tulapurkar, A. A. et al. Spin-torque diode effect in magnetic tunnel junctions. Nature 438, 339-342 (2005).